Food items such as poultry, fresh red meat and cheese, as well as nonfood industrial and retail goods, are packaged by various shrink wrap methods. Shrink wrap films can be monoaxial or biaxial oriented and are required to possess a variety of film attributes.
There are two main categories of heat shrink films—hot-blown shrink film and oriented shrink wrap film. Hot-blown shrink film is made by a hot-blown simple bubble film process and, conversely, oriented shrink wrap film is made by elaborate biaxial orientation processes known as double bubble, tape bubble, trapped bubble or tenter framing. Both amorphous and semi-crystalline polymers can be made into oriented shrink wrap films using elaborate biaxial orientation processes. For amorphous polymers, the orientation is performed at a temperature immediately above the glass transition temperature of the polymer. For semi-crystalline polymers, the orientation is performed at a temperature below the peak melting point of the polymer.
shrink packaging generally involves placing an item(s) into a bag (or sleeve) fabricated from a shrink wrap film, then closing or heat sealing the bag or sleeve, and thereafter exposing the package to sufficient heat to cause shrinking of the film and intimate contact between the film and item. The heat that induces shrinkage can be provided by conventional heat sources, such as heated air, infrared radiation, hot water, hot oil combustion flames, or the like. Heat shrink wrapping of food items helps preserve freshness, is attractive, is hygienic, and allows closer inspection of the quality of the packaged food item. It is beneficial to expose perishable food to the lowest temperature necessary to induce film shrinkage. Additionally, inducing shrinkage at the lowest possible temperature reduces manufacturing costs. Heat shrink wrapping of industrial and retail goods, which is alternatively referred to in the art and herein as industrial and retail bundling, preserves product cleanliness and also is a convenient means of bundling and collating for accounting and transporting purposes.
The biaxial heat-shrink response of shrink film is obtained by initially stretching fabricated film to an extent several times its original dimensions in both the machine and transverse directions to orient the film. The stretching is usually accomplished while the fabricated film is sufficiently soft or molten, although cold drawn shrink films are also known in the art. After the fabricated film is stretched and while still in a stretched condition, the stretching or orientation is frozen or set by quick quenching of the film. Subsequent application of heat will then cause the oriented film to relax and, depending on the actual shrink temperature, the oriented film can return essentially back to its original unstretched dimensions, i.e., to shrink relative to its stretched dimension.
In the past, it was believed that the shrink temperature of polyolefin films was based on a linear function of density. For example, U.S. Pat. No. 6,306,969 teaches that polyolefin film shrinkage was a function of shrink tension and film density. However, applicants now believe, without wishing to be bound by the theory that shrink temperature is directly proportional to the Vicat softening point, which is nonlinearly proportional to the density of the polyolefin blend. Nonlinearity allows depressing softening temperatures while maintaining performance and cost competitiveness.
It would be beneficial to provide a film having a relatively high toughness i.e., relatively high crystallinity, and having a reduced film orientation temperature and reduced shrink temperature. In this manner, a reduced amount of heat would be necessary to shrink wrap products, while maintaining film toughness. The reduced heat translates into reduced energy cost as well as reducing the amount of heat to which the contents of the film are exposed.
There exists a need for polyethylene compositions, for use in shrink wrap products, having reduced shrink temperature.